Most modular systems require a controller or control interface. 

This is the case for the VME, VXI, PXI, VPX, EtherCAT … systems.

The controller or the control interface ensures the communication and management of the various cards integrated in the chassis. The controller can be an intelligent module, integrating a PC function and in this case the program of the system application will reside on the module. Outward communication will be via conventional interfaces such as Ethernet. The system with intelligent controller can thus operate in stand-alone mode.

The controller can be a simple control interface and in this case will require the use of a remote PC. The interface will communicate in this case via a standard bus protocol such as PCI Express, StarFabric, Ethernet, USB, … or via a proprietary bus protocol.